THE EVENTS IN HUMAN REPRODUCTION

Gametogenesis Insemination Fertilization Implantation Gestation Parturition

FORMATION OF GAMETES

Sexual reproduction requires the fusion of two haploid gametes to form a diploid individual. These haploid cells are produced through gametogenesis.

As there are two types of gametes, the spermatozoa and ova, gametogenesis can be studied under two broad headings: spermatogenesis and oogenesis.

Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova. Both spermatozoa and ova originate from primordial germ cells or PGCs, which are extra-gonadal in origin.

In humans, the PGCs originate during early embryonic development from the extra-embryonic mesoderm. Eventually, they migrate to the yolk sac endoderm, and ultimately, to the gonads of the developing embryo, where they undergo further development.

SPERMATOGENESIS

Spermatozoa are produced in the seminiferous tubules of the testes. Spermatogenesis is the process of maturation of reproductive cells in the testes.

Spermatogenesis includes two stages (i) formation of spermatids and (ii) metamorphosis of spermatids. Spermatids are formed by three phases namely phase of multiplication (mitosis), phase of growth and phase of maturation (meiosis).

During phase of multiplication, the primordial germ cells divide repeatedly by mitosis to form diploid spermatogonia.

During phase of growth, the spermatogonium enlarges in size to form primary spermatocyte and prepares to undergo maturation division.

During phase of maturation, the primary spermatocyte undergoes meiosis I giving rise to two haploid (n) secondary spermatocytes. The secondary spermatocytes undergo meiosis II resulting in the formation of four spermatids.

Transformation of spermatid to sperms is termed spermiogenesis. A spermatid is non-motile. It has organelles like mitochondria, Golgi bodies, centrioles, nucleus etc.

During spermiogenesis, the weight of gamete is reduced along with development of locomotory structures. Nucleus becomes compact forming the major part of head of spermatozoa,

Golgi complex of spermatid give rise to acrosome. Acrosome forms a cap in front of nucleus containing lytic agent which dissolves egg membranes during fertilization.

Acrosome of mammalian sperm produces sperm lysins. The two centrioles of the spermatids become arranged one after the other behind the nucleus. The anterior one is known as the proximal centriole.

The proximal centriole is usually located on the neck of spermatozoan. During fertilization, it is introduced in to the egg and is required for the first cleavage.

The posterior centriole is known as the distal centriole and gives rise to the axial filament of the sperm. Mitochondria from different parts of spermatid to get arranged in the middle piece around the axial filament.

Mitochondria in the middle piece provide energy to the sperm for locomotion.

A typical mammalian sperm is flagellated, consisting of four parts namely head, neck, middle piece and tail.

The human sperm was first seen by Hamm and Leeuwenhoek. Tail-less, (non-flagellate) 'amoeboid' sperm is found in the roundworm Ascaris.

Hormonal control of spermatogenesis

Spermatogenesis is under the control of endocrine hormones. Hypothalamus produces gonadotropin releasing hormone or GnRH.

It acts on anterior pituitary to produce gonadotropins, ICSH and FSH, ICSH interstitial cell stimulating hormone acts on interstitial or Leydig cells which produce testosterone.

Testosterone is essential for formation of sperms, atleast spermiogenesis part by Sertoli cells. Under the influence of FSH, Sertoli cells develop androgen binding protein (ASP).

The latter helps in concentrating testosterone in the seminiferous tubules.

Excess of testosterone inhibits LH/ICSH by anterior pituitary and GnRH production by hypothalamus.

Sertoli cells also produce a glycoprotein called inhibin. Inhibin suppresses FSH synthesis by anterior pituitary and GnRH synthesis by hypothalamus.

Thus normal release of testosterone is under negative feed back control.

OOGENESIS

Oogenesis is the process of maturation of reproductive cells in ovary. Oogenesis starts before birth. In 25 weeks old female foetus, all the oogonia are produced.

Oogenesis is basically similar to spermatogenesis. It includes phase of multiplication, phase of growth and phase of maturation.

During the phase of multiplication, the primordial cells in the ovary divide mitotically to form oogonia (egg mother cell). Each oogonium divides mitotically to form two primary oocytes.

Primary oocytes undergo growth, the growth phase during oogenesis is comparatively longer.

Primary oocytes, begin the first step of Meiosis-I and proceed up to diakinesis.

These oocytes resume their development at puberty. The primary oocyte (2n) completes meiosis-I producing two haploid cells (n), the larger one is secondary oocyte and the smaller one is first polar body.

Secondary oocyte starts Meiosis-II and proceeds upto metaphase-II only. Further development will start only after arrival of spermatozoa.

Entry of sperm restarts the cell cycle by breaking down M PF (M-phase promoting factor) and turning on APC (Anaphase promoting complex). Completion of meiosis II results in the formation of functional egg or ovum and a second polar body.

Compare the structure of Mammalian sperm and Mammalian ovum

 

Schematic representation of (a) Spermatogenesis; (b) Oogenesis

Differences between Spermatogenesis and Oogenesis

  Hormonal control of Oogenesis

In response to production of GnRH or gonadotropin releasing hormone, anterior pituitary secretes two hormones, FSH (follicle stimulating hormone) and LH (luteinizing hormone).

FSH stimulates follicular growth and maturation of oocyte. Granulosa cells of developing ovarian follicle produce estrogen.

In presence of high titre of both estrogen and LH, ovulation occurs. High concentration of estrogen inhibits secretion of both FSH and GnRH.

This is negative feedback control LH helps in converting ruptured Graafian follicle into corpus luteum.

The latter secretes progesterone which prepares the uterus to receive fertilised ovum. High concentration of progesterone inhibits further release of LH from anterior pituitary and GnRH from hypothalamus.